The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
GMR stack 11 is schematically shown in FIG. 1. Its key elements (not shown) are an antiferromagnetic layer whose purpose is to act as a pinning agent for a magnetically pinned layer. Lying on the latter is a copper spacer layer on which there is a low coercivity (free) ferromagnetic layer. Magnetic shield layers 12 and 13 lie immediately above and below the spin valve and serve to block out external magnetic influences that might upset the operation of the unit, while leaving it free to interact with magnetic fields above the plane of the figure (i.e. from the storage media).
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. The magnetization of the free layer will be in a direction dictated by the minimum energy state. This, in turn, is determined by a number of factors including the crystalline and shape anisotropies.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a CIP spin valve is typically 8–20% while for a CPP SV this value can be over 60%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have begun to emerge. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible. In contrast, in a CPP device, the total transverse resistance of all layers, other than the free layer, should be as low as possible so that resistance changes in the free layer can dominate.
Since the two shields, 12 and 13, are made of soft magnetic materials they will sense the bit fields from the disk media by its AMR effect as sense current flow (ΔRAMR) in the shields. ΔRAMR constitutes noise that is super-imposed on the GMR signal. This phenomenon is illustrated in FIG. 2, where an R-H curve of a CPP shield is shown. Note the portion within circle 21. The part of the curve near zero field that has gone negative represents AMR pickup by the shield.
It is the purpose of the present invention to reduce this noise by 5 to 10 times. A method for achieving this is disclosed below.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,515,573, Bong et al. describe a small free layer resulting in reduced noise. Smith et al. discuss noise in the free layer in U.S. Pat. No. 6,473,279. U.S. Pat. No. 6,469,879 (Redon et al) shows that noise occurs if the free layer is too thin. U.S. Pat. No. 5,959,811 (Richardson) discloses noise current in the leads while Rottmayer et al. teach a configuration to reduce noise in the read signal in U.S. Pat. No. 5,784,224.